Osteoclastic bone resorption mediated by inflammatory cytokines is an important contributor to morbidity in patients with rheumatoid arthritis. Development of agents that could prevent or alleviate this bone loss would represent an important clinical advance. Screening of natural products has revealed several classes of agents that would be potentially effective as new anti-resorptive agents due to their ability to interfere with critical steps in osteoclast differentiation and action. The two compounds that would be investigated in the current proposal, compactin and reveromycin A, inhibit osteoclast activity. Compactin prevents osteoclast differentiation by inhibiting fusion of preosteoclasts mediated by Osteoclast Differentiating Factor (ODF) and Macrophage- Colony Stimulating Factor (M-CSF). It also disrupts the actin rings that are crucial to the resorptive activity of mature osteoclasts. Compactin is an HMG-CoA reductase inhibitor and its effects on osteoclasts appear to involve the decreased production of isoprenyl moieties through this pathway. Small G-proteins are critical targets for activation by isoprenylation, and this activation is critical for cytoskeletal function. The current proposal would determine the G-protein targets that are affected by the inflammatory cytokines and compactin in two osteoclast models, osteoclasts deriving from marrow/osteoblast co- cultures and the RAW 264.7 osteoclastogenic macrophage cell line and determine whether the inhibition of their isoprenylation is the mechanism of the inhibitory effect of compactin. Reveromycin A targets are less well defined, but may involve increased osteoclast apoptosis through activation of caspase 3. The studies would determine if this is a potential site of interaction between reveromycin and cytokines, and would identify other potential pathways and target molecules. Thus, the current proposal would help to elucidate the molecular mechanisms by which the actions of compactin and reveromycin A interfere with osteoclastic activity. Elucidation of the mechanism of these compounds could lead to a greater understanding of pathological bone loss and could result in the development of effective new therapeutic agents.